1. Field of the Invention
The present invention relates to three-dimensional expanded polyurethane foam materials, such as flexible reticulated polyurethane foam compositions, that are electrically conductive and have antistatic properties. More specifically, the invention relates to a polyurethane foam having long-lasting and reliable electrical conductivity characteristics and a volume resistivity of approximately 10.sup.12 ohm.multidot.cm or less at room temperature (about 70.degree. F.). This foam is produced by combining conventional polyurethane foam-forming reactants and an effective amount of an antistatic agent, such as tetracyanoquinodimethane (TCNQ) or sodium perchlorate, under foam-forming conditions. In one preferred embodiment, the conductive foam is subsequently reticulated by momentary exposure to a flame front.
2. Description of the Prior Art
It has long been known that it is not possible to generate an explosion of any fuel/air mixture in the presence of reticulated polyurethane foam structures. Because of this characteristic of reticulated polyurethane foam and the additional benefits of light weight, minimal reduction in a fuel tank's usable space and attenuation of violent surging of fuel, reticulated polyurethane foam structures have found extensive use in military aircraft and land vehicles. These properties reduce the vulnerability of these vehicles by providing protection against explosive ballistic hits, explosions following fuel tank rupture, explosive static discharge and explosions caused by lightning.
Unfortunately, the use of foam can give rise to a different danger. Since the foam has a low electrical conductivity (or as more conveniently measured, a high electrical resistivity), there can be a buildup of electrostatic charges and the electrostatic charges can result in sparking, leading to a fire or explosion within the non-foam containing areas of the fuel tank. The build-up of electrostatic charges can also adversely effect sensitive aircraft instrumentation.
Consequently, it is an object of this invention to provide a foam for fuel tanks which minimizes the possibility of fuel explosion from gunfire, electric ignition and lightning. It is a further object of this invention to provide a foam which has a higher electrical conductivity needed to minimize the danger of sparking caused by the build-up of electrostatic charges. It is a further object of this invention to provide fuel tanks which minimize the release of fuel in the event of rupture and which resist sparking due to the build-up of electrostatic charge.
U.S. Pat. No. 4,886,626 (Cope et al.) describes a conductive polyurethane foam which contains a charge transfer agent selected from the group consisting of tetracyanoethylene (TCNE), picric acid and analogs thereof. These foams exhibit electrical resistivities (i.e., volume resistivities) of 10.sup.12 ohm.multidot.cm or less. However, these foams suffer from the disadvantage that the charge transfer materials used in the foams are dangerous to handle. For example, TCNE is highly toxic and picric acid and its derivatives are explosive.